The present holographic scanning telescope represents a brand new technique in remote sensing instrumentation, and an innovative application of a technology that has been developing along for the past twenty some years. Recent developments in HOE's using volume phase holograms enables the development of a new class of light-weight telescopes with specific advantages for lidar remote sensing instruments. In particular, a large aperture, narrow field of view telescope used in a conical scanning configuration can be constructed with a much smaller rotating mass compared to conventional designs which require scanning either the entire telescope assembly including the primary mirror, secondary, baffling, and the detector package or rotating a separate flat scanning mirror which is larger in size than the primary collection optic. Because laser remote sensing systems utilize a spectrally narrow wavelength band or bands and require optical blocking and interference filters to reject unwanted wavelengths, especially scattered solar radiation, the holographic telescope can be made to diffract only a narrow wavelength band or bands thus reducing or even eliminating the requirements for optical blocking and interference filters.
An HOE is essentially a hologram of a lens or curved mirror. It is a diffractive optic that has optical power, that is, the ability to form images. The hologram itself usually consists of a film emulsion containing a diffraction pattern as a surface relief or as index modulation throughout the thickness of the film as in the volume phase hologram. It may be fixed to a planer substrate or to one that has curvature contributing to the optical power. Being a diffractive optic, an HOE has spectral dispersion which can be utilized to advantage in a lidar system in which only one or more particular laser wavelengths are being observed. The device described herein may also be applied to advantage in passive instruments requiring spectral filtering or dispersion.